1. Field of the Invention
The invention is directed to veneer ceramics for dental restorations in which the framework ceramic is made of yttrium-stabilized zirconium dioxide.
2. Description of the Related Art
Yttrium-stabilized zirconium dioxide is a high-performance material of extremely high strength that is used to an increasing extent in restorative dentistry for framework ceramics for crowns, inlays, and bridges. The application of veneer ceramics is required for fine adjustment to the diversity of natural teeth. Up to the present, veneer ceramics have presented a weak point in the ability of the restored teeth to withstand stress.
Veneer ceramics should permit excellent shaping, be conformable to the adjacent teeth with respect to coloring, be highly resistant to chemicals, have a high flexural strength even after a directed heat treatment, and be characterized by outstanding adhesion to the framework ceramic.
Powders or pastes are generally used as starting materials for producing the veneer ceramics. The properties of the veneer ceramic are determined by the chemical and crystallographic features as well as by the grain size of the starting materials.
According to U.S. Pat. No. 4,798,536 A, leucite-containing dental porcelains are produced by means of the fused glass. The leucite content is in the range of 35 to 60 percent by weight. The high coefficient of expansion of the leucite-containing dental porcelain of 13 to 15×10−6/K is used for veneering metal crowns. The flexural strength of the veneer ceramic with leucite crystals is 80 MPa.
The use of lithium disilicate is proposed for a restorative tooth prosthesis in U.S. Pat. No. 4,189,325 A. This reference concentrates on the material system of Li2O—CaO—Al2O3—SiO2. The nucleating agents Nb2O5 and Pt are added to promote crystallization.
U.S. Pat. No. 4,515,634 A suggests the addition of the nucleating agent P2O5 to the basic system of Li2O—CaO—Al2O3—SiO2 in order to improve nucleation and crystallization.
Laid Open Application DE 197 50 794 A1 describes the use of lithium disilicate glass ceramics use in the hot pressing method. However, it has been shown that application of this method results in insufficient edge strength of the restored tooth and increased tool wear during finishing.
DE 103 36 913 A1 suggests a two-stage fabrication of the tooth to be restored. In the first step, lithium metasilicate is crystallized and is mechanically worked to form dental products. The lithium metasilicate is converted to the stronger lithium disilicate by a second heat treatment. Accordingly, the restored tooth is made entirely of glass ceramic with lithium disilicate crystals.
German Patent DE 196 47 739 C2 describes a sinterable lithium disilicate glass ceramic and glass. The starting material is sintered to form blanks. These blanks are pressed at 700° C. to 1200° C. to form dental products. The described lithium disilicate glass ceramic shows only a slight reaction to the adjacent casting investment during plastic deformation.
EP 1 235 532 A1 describes a method for producing a high-strength ceramic dental prosthesis based on yttrium-stabilized zirconium dioxide. The framework ceramics produced by this method have 4-point flexural strengths greater than 1200 MPa.